Wide-range variable delay line and ring oscillator

Abstract: A variable delay line (10) having a string of "slow" logic inverters (11) and an equal number of "fast" inverters (14) with inputs connected to corresponding "slow" inverter inputs. Transmission gates (15), coupling the "fast" inverter outputs to corresponding "slow" inverter outputs, vary the amount of coupling between the inverters. Maximum delay occurs with substantially no coupling between the "fast" and "slow" inverters and minimum delay occurs when maximum coupling occurs between the "fast" and "slow" inverters. The variable delay line may be configured into a variable frequency ring oscillator, useful in phase-locked-loops or the like.

Description: This invention relates to variable delay lines. Delay lines are widely used in a variety of integrated circuit applications, such as delay-locked-loops and clock generators. The delay line is typically formed on an integrated circuit by cascading a string of logic inverters, each connection between the inverters serving as a "tap" along the delay line. One method of controlling the amount of delay through the line is by varying ("starving") the power supply current supplied to the inverters. Another way is by varying the amount of capacitance loading on the output of the inverters. A drawback to the above delay control approaches is the possible inability of the delay line delay to be as short as the technology used to implement the delay line allows. For example, the minimum delay for a current "starved" inverter based delay line may be significantly longer than if the inverters were not current limited. Further, at maximum delay, the signals propagating through the inverters may be so slow in rise and fall times that instabilities in circuits coupled to the delay line may occur. Connecting the input of an active delay line to its output forms a ring oscillator which are widely used in, for example, integrated circuit phase locked loops. The oscillators generally have an odd number of inverting delay stages (typically logic inverters) serially connected into a ring with the oscillation frequency being substantially determined by the inverter delays. The amount of delay by each inverter is controlled as discussed above. In correspondence with the variable delay line problems mentioned above, while the ring oscillator does allow for wide variations in oscillation frequency, the highest oscillation frequency may be considerably less than the maximum possible for the logic technology used to implement the oscillator. Further, the lowest frequency of oscillation may not have sufficient amplitude for circuits utilizing the output of the oscillator and the oscillator may completely stop oscillation due to the gain of the inverters being insufficient to support oscillation. Still further, the relationship between the control signal and the oscillation frequency may be very non-linear. Thus, it is desirable to provide a variable delay line design having wide delay range without having too slow logic rise and fall times at the longest desired delays and having shorter minimum delays than prior art designs. Further, it is desirable to provide a ring oscillator design that has wide frequency variation range, a higher maximum oscillation frequency than prior art designs, and consistent, full amplitude, reliable oscillation at the lowest desired frequency of oscillation. In addition, it is desirable to provide an oscillator with an approximately linear control signal to oscillator frequency relationship. According to this invention there is provided a variable delay line as claimed in claim 1. The invention will now be described with reference to the accompanying drawings, in which: FIG. 1 is a simplified schematic diagram of an exemplary embodiment of the invention as a voltage controlled variable delay line; FIG. 2 is a simplified schematic diagram of an exemplary logic inverter; FIG. 3 is a simplified schematic diagram of a transmission gate used in the exemplary embodiment of FIG. 1; FIG. 4 is an exemplary plot showing the relationship between a control voltage and the oscillation frequency of the oscillator in FIG. 1; and FIG. 5 is a simplified schematic diagram of an exemplary embodiment of the invention as a voltage controlled ring oscillator in a phase-locked-loop. Detailed Description Referring to FIG. 1, an exemplary embodiment of the invention as a variable delay line 10 is shown as a three stage (logic gate) delay line having a string of relatively long propagation delay ("slow") logic gates 11 and paralleled corresponding relatively short propagation delay ("fast") logic gates 14. Transmission gates 15, responsive to control signals VCN, VCP, vary the amount of coupling between "slow" logic gates 11 and corresponding "fast" logic gates 14 to vary the delay through the delay line 10. Two outputs are provided, OUT(DELAY) 16 and OUT(RING OSC.) 17. The output 16 has the fastest rise and fall times because it is being driven by a fast inverter 14. Since it is desirable to have subsequent logic circuitry (not shown) driven by logic signals having the fastest rise and fall times, the output 16 is used for driving additional logic. However, because the output 17 has relatively longer rise and fall times than the output 16 particularly at longer delays through delay line 10, this output is useful in ring oscillator applications, as will be discussed below. The logic gates 11, 14 are shown as common CMOS inverters, such as that shown in FIG. 2. The "slow" inverters 11 have the transistors therein sized to provide relatively low output current, thus determining the longest delay in the delay line 10(FIG. 1). In contrast, the "fast" inverters 14 are designed to have the transistors therein sized to provide a large amount of current and are typically sized to have the shortest propagation delay for the technology used. For example, for the "slow" inverters 11, the N-channel and P-channel transistors are made with an exemplary length-to-width ratios of 10:1 and 20:1, respectively, the delay of the inverter 11 is approximately 10 ns when the output thereof is loaded with and exemplary load capacitance of approximately 0.1 pF, representative of the minimum loading of a typical inverter 11 as shown in FIG. 1. For the "fast" inverter 14, the exemplary length-to-width ratios of the N-channel and P-channel transistors are 3:10 and 6:10, respectively, resulting an exemplary propagation delay of approximately 1 ns when the inverter is loaded with an exemplary capacitive load of approximately 0.25 pF, representative of the maximum loading of an exemplary inverter 14 in FIG. 1. As shown in FIG. 3, the transmission gates 15 are of a conventional design having an N-channel MOSFET transistor and a P-channel MOSFET transistor with common drains and sources. The gates thereof couple to the respective control signals VCN, VCP which control the conductivity of the corresponding transistors in the transmission gate 15 and, consequently, the total delay of the delay line 10 (FIG. 1). An exemplary relationship between the control signals VCN and VCP is shown in FIG. 4. For purposes here, the signals VCN and VCP are complementary: as the voltage VCN increases, the voltage VCP decreases. Thus, the conductivity of both the N-channel and P-channel transistors in the gate 15 (FIG. 3) change concurrently. Further, it is preferable that the conductivity of the N-channel transistor and the P-channel transistor be substantially the same. As shown, as the control signals change to increase the conductivity of the gate 15, the delay of the delay line 10 (FIG. 1) decreases. By taking the delay line 10 (FIG. 1) and coupling the input to the output 17 thereof, a variable frequency ring oscillator 22 is formed as shown in FIG. 5. Output of the ring oscillator 22 is taken from the output 16. The oscillation frequency of the ring oscillator 22 is substantially determined by the delay through the delay line 10. Advantageously, because the minimum frequency of the ring oscillator is substantially determined by the maximum delay through the delay line 10, reliable oscillation of the ring oscillator occurs at the minimum frequency. Further, the maximum frequency of oscillation occurs when the delay line 10 is at minimum delay. By adding a conventional phase detector 21 (e.g., a charge pump phase detector with complementary outputs) to compare the output frequency of the ring oscillator 22 to a reference signal and to control the frequency of the ring oscillator 22, a phase-locked-loop 20 is formed. EXEMPLARY RESULTS A ring oscillator 22 has been formed on an integrated circuit using the circuit substantially as shown in FIG. 1 as the variable delay line 10. Using the exemplary transistor sizes discussed above in a 0.9 µm CMOS process, the ring oscillator 22 has an approximate frequency range of 10 to 100 MHz. It is understood that other circuit topologies and technologies instead of CMOS may be used to implement the delay line 10. The number of inverter stages in the delay line may be increased or decreased as needed. It is understood that an odd number (three or more) inverter stages are needed for the ring oscillator 22 as shown in FIG. 5. Further, the logic gates 11, 14 may be more complex logic functions such as NAND or NOR gates. Still further, all the logic gates need not be inverting.

Claims: A variable delay line (10) having an input, an output and a string of directly connected first logic gates (11) ) connected between the input and the output of the delay line, each of the first logic gates having an input and an output, CHARACTERIZED BY: a plurality of second logic gates (14), each gate having an output and an input, the input coupling to a corresponding input of a first logic gate; and a plurality of means (15) for variably coupling the outputs of the second logic gates to corresponding outputs of the first logic gates in response to a control signal (VCP, VCN); wherein the first logic gates have delays longer than the delays of the second logic gates, and the control signal substantially determines the delay of the delay line. A variable delay line as claimed in claim 1, wherein each of the plurality of means is a transmission gate. A variable delay line as claimed in claim 2, wherein the logic gates are inverters. A variable delay line as claimed in claim 2, wherein the logic gates are CMOS inverters, and the transmission gates are complementary MOSFETs with common sources and drains. A variable delay line as claimed in claim 3, wherein the input of the delay line is coupled to the output thereof to form a ring oscillator. A variable delay line as claimed in claim 5, including a phase detector (21), coupling to the output of the delay line and responsive to a reference signal (REF), for generating the control signal.

Inventors: LEONOWICH ROBERT HENRY, LEONOWICH, ROBERT HENRY
Applicants: AT & T CORP, AT&T CORP.
Application year: 1994
Main classification: H03K5
IPC codes: H03K17, H03K3, H03L7, H03K5